Comparison of the deduced amino acid sequences of three viral myc sequences (from avian acute leukemia viruses MC29, MH2, and OK10) with cellular proto-myc sequences from trout, chicken, mouse, and human cells indicates that there is a single position where each of the viral sequences differs from the cellular sequences. This position and its neighbors are completely conserved in the vertebrate sequences examined. Since this is unlikely to have occurred by chance alone, the mutations in this position may influence the oncogenic potential of the viruses. A simultaneous comparison of the trout, chicken, and human cellular myc sequences indicated that while on the whole these sequences are highly conserved, each sequence contains unique regions that likely were introduced by insertion events. The genome of the AIDS virus, HTLV-III, contains an open reading frame termed sor that is conserved among different isolates of the virus. Antibodies raised against a bacterially-expressed protein containing sor sequences precipitated an Mr 23,000 protein. This protein comigrated with protein precipitated by some sera of HTLV-III-infected humans and, thus, is likely the product of the viral sor region. When radiolabeled HTLV-III-infected T-cells were fractionated and the fractions assayed for sor by radioimmunoprecipitation, the sor protein was determined to be predominantly located in cytoplasm.